EDIT - replacing the blown rear right lower brake Light did NOT solve it...:
It was as simple as one of the normal rear brake lights. When I previously inspected myself against a large supermarket window I did not notice that the car is so smart that it uses one of the rear fog (and also panic brake) lights if the brake light bulb is blown. So much for tearing down the tail lid and checking all bulbs at the back. I did check the brake light bulb but failed to see that is had gone. When I checked again by backing up to a large window I saw that the brake light was assymretrical. Case closed and one hour wasted

Now I have the same problem. I saw this a long while ago when the bulb holder for one of my two rear fog lights had been a bit burnt where the bulb socket meets the holder. Scratched that away and was all good. Now 1½ years later I *think* I have checked all bulbs but still have a warning.